1. Field of the Invention
The present invention relates to a semiconductor circuit utilizing a crystalline silicon film formed on a quartz substrate or the like and, more particularly, to a semiconductor circuit having the function of an operational amplifier.
2. Description of the Related Art
Recently, researches are being carried out on techniques for forming semiconductor devices utilizing a crystalline silicon film on a quartz substrate in an integrated manner. A typical example of such techniques is a technique for providing an active matrix circuit and a peripheral driving circuit for driving the same circuit on a single quartz substrate or glass substrate.
The required circuit configurations include active matrix circuits, shift register circuits and buffer circuits.
An active layer of a thin film transistor forming a part of a circuit is formed using a crystalline silicon film. A crystalline silicon film can be fabricated by forming an amorphous silicon film on a substrate and then heating the same or irradiating the same with laser beams or performing both to anneal the same.
A thin film transistor having an active layer formed by a crystalline silicon film is more excellent in characteristics such as mobility than those having an active layer formed by an amorphous silicon film.
There is a need for higher levels of integration and higher performance also for circuits formed using thin film transistors.
Recently, it is contemplated to use thin film transistors to configure, on a substrate, not only logic circuits such as shift registers but also circuits having computing functions such as operational amplifiers which have conventionally been externally attached to a substrate.
Operational amplifier circuits have been generally configured using a single crystal silicon wafer.
An operational amplifier is basically comprised of a differential amplifier circuit. A differential amplifier circuit is formed by combining two transistors having similar characteristics.
In the case of a differential amplifier circuit, a change in temperature or power supply voltage affect the two transistors simultaneously. Therefore, a change in temperature or power supply voltage does not affect the output of the same.
In order for this, the two transistors forming the differential amplifier circuit must have similar characteristics.
In practice, since it is difficult to provide two transistors having completely identical characteristics, efforts are being made toward manufacturing techniques to provide transistors as much similar to each other as possible in their characteristics.
Thin film transistors utilizing a crystalline silicon film have mobility lower than that of MOS transistors fabricated using a single crystal silicon wafer. Further, they have a higher level of variation in characteristics.
For this reason, it has been difficult in practice to form an operation amplifier circuit using such thin film transistors.
The present invention solves this problem. Specifically, it is an object of the invention to form a practical operational amplifier circuit using thin film transistors.
According to an aspect of the invention, there is provided a group of operational amplifier circuits constituted by thin film transistors formed on an insulating surface characterized in that:
the operational amplifier circuits comprise a combination of at least n-channel type thin film transistors and p-channel type thin film transistors;
Cumulative distribution of mobilities of the n-channel type thin film transistors becomes 90% or more at 260 cm2/Vs; and
cumulative distribution of mobilities of the p-channel type thin film transistors becomes 90% or more at 150 cm2/Vs.
The above-described structure is formed on an insulating substrate represented by a quartz substrate. The use of a substrate having insulating properties makes it possible to configure a circuit suitable for operations at high speeds because it eliminates effects of capacitance of a substrate. According to another aspect of the invention, an active layer of a thin film transistor is formed by a crystalline silicon film having a structure in which a multiplicity of columnar crystal structures extend in a direction that matches the moving direction of carriers.
According to the invention, since a thin film semiconductor is used for the active layer, the source and drain regions can be activated (after doping) by irradiating them with laser beams or intense beams.
This allows the use of aluminum which is a low-resistance material or a material mainly composed of aluminum for the gate electrode to improve adaptability to high speed operations.
Further, since the unique crystal structure suppresses the short-channel effect, predetermined operational performance can be achieved with dimensions larger than dimensions indicated by conventional scaling rules.
For example, when the above-described crystalline silicon film is used, a gate insulation film having a thickness on the order of 500 xc3x85 provides characteristics that have been available with only a gate insulation film having a thickness on the order of 200 xc3x85 according to conventional scaling rules.
It is technically and economically difficult to form a thin gate insulation film having preferable interface characteristics, no pin hole and a high withstand voltage over a large surface area.
From this point of view, it is advantageous to achieve predetermined characteristics free of limitations placed by conventional scaling rules.
In addition, the average S-value of thin film transistors utilizing a crystalline silicon film having the above-described unique crystal structure can be 100 mV/dec or less for either of p- and n-channel type thin film transistors even when a multiplicity of the same are formed on a substrate.
The S-value of a TFT fabricated using a general high temperature process (a general term that refers to processes for fabricating a TFT on a quartz substrate using an annealing step at about 1000xc2x0 C.) is about 200 mV/dec or more when it is an n-channel type TFT and about 350 mV/dec or more when it is a p-channel type TFT.
The S-value of a TFT fabricated using a low temperature process (a general term that refers to processes for fabricating a TFT on a quartz substrate using a laser annealing step) is worse than that of a TFT fabricated using a high temperature process.
According to another aspect of the invention, there is provided a group of operational amplifier circuits constituted by thin film transistors formed on an insulating surface characterized in that:
the operational amplifier circuits comprise a combination of at least n-channel type thin film transistors and p-channel type thin film transistors; and
active layers of the thin film transistors comprise a crystalline silicon film having a structure wherein a multiplicity of columnar crystal structures extend in a direction that matches the moving direction of carriers.